Conventionally, in fabricating MEMS dies for use in the electronics industry, a MEMS wafer is typically fabricated which comprises a plurality of individual MEMS dies commonly arranged in a grid pattern. The sections of a wafer between the individual dies are termed scribe streets.
Scribe streets are areas of the wafer where no component has been placed and which define the boundaries of each individual MEMS die. The MEMS structures appear on only one surface of the wafer. The opposing surface is devoid of structures or patterns. The individual MEMS dies comprising a MEMS wafer are removed from the wafer by sawing through the MEMS wafer along all of the streets, thus physically separating the wafer along both axes into the individual MEMS dies.
One example of standard industry practice for separating wafers into individual dies is described below. First, the wafer is placed upside down (i.e., with the patterned side of the wafer facing downwardly and the non-patterned side facing upwardly) on a flat surface. A metal film frame defining an opening is laid over the wafer with the wafer being within the perimeter of the opening in the film frame. A plastic film is then laid over the metal film frame and back (non-patterned side) of the wafer. Preferably, the plastic film is coated with adhesive on the side that contacts the film frame and the back of the wafer.
Force is then applied between the film and the film frame to cause the film to adhere well to the frame. One possible technique for applying the force is to pass a rolling cylinder over the wafer and film frame to adhere the film to the backside of the wafer and to the surface of the film frame. The wafer is now mounted to the film that, in turn, is mounted to the film frame.
The wafer, film, and film frame combination (hereinafter the “film frame assembly”) is then turned over so that the patterned side of the wafer now faces upwards. The film frame assembly is then placed on a movable chuck in a sawing station. The sawing machine typically comprises a camera and a computerized optical system utilizing optical pattern recognition software with movement of the chuck so as to align the streets on the wafer with the saw blade.
This can also be done manually by observing a video image obtained by the camera on a screen and manually adjusting the position of the pallet to the desired location. The chuck and wafer is then advanced under the saw blade to cut through the streets.
Commonly, a wafer has a first plurality of parallel streets aligned in a first direction and a second plurality of parallel streets are aligned orthogonal to the first plurality of streets thus defining a grid with individual dies comprising the blocks between the orthogonal streets. Accordingly, the wafer will be advanced through the saw blade to cut along a street, shifted laterally to the cutting direction a distance equal to the spacing between the parallel streets and advanced through the saw blade to cut the next street. This process is repeated until all of the first plurality of parallel streets are cut.
The chuck and wafer are then rotated 90° and the wafer is advanced through the blade a number of times again to cut through all of the parallel streets in the second orthogonal direction. The saw blade height is adjusted such that it will cut completely through the wafer and contact and score, but not cut through, the film.
The plastic film may be a polyolefin or poly vinyl chloride film of approximately 3 mils in thickness. The blade height would be set, for instance, to cut 1.5 mils into the film. During the sawing process, water is jet sprayed over the surface of the wafer as well as over the surface of the saw blade to cool the wafer and saw blade. After the sawing operation, the wafer is transported to a cleaning station where it can be sprayed with de-ionized water and brushed to clear away any remaining silicon slurry created by the sawing operation.
The wafer typically is then dried after the water flow and brushing operations are completed. The drying may be accomplished in the cleaning station by rapid rotation of the wafer or, alternatively, the wafer may be removed to an oven for heat drying. Other drying options are also available.
After cleaning, the film frame assembly is transported to a pick-and-place station where the now detached individual dies are to be removed from the film. The pick-and-place station removes the individual dies from the film and places them, for instance, in a carrier. Commonly, the film frame assembly (to which the individual dies are still adhered) is slid into a movable slotted holder in the pick-and-place station that is located above an anvil comprising a needle or needle cluster.
A camera is positioned above the anvil and the film frame assembly to obtain an image of a die on the film frame assembly. The image is processed in an optical pattern recognition system and the position of the film frame assembly is adjusted to line up a die with the anvil. The film frame assembly is then clamped in place and a mechanism grasps the film beyond the perimeter of the wafer and stretches the film radially outward. The stretching of the film serves to reduce the film adhesion to the dies at the edge of the dies, and to create space between the dies. After the stretching operation, the anvil is used to further separate the dies from the film.
The anvil contains a needle or needle cluster that is advanced upwardly to contact the film underneath the selected die, pierce the film and push the die upwards. Also under control of the computer and pattern controls recognition software, an arm having a vacuum-equipped probe is positioned over the top surface of the die. The arm lowers the probe into contact with the die and the vacuum pressure causes the die to attach to the probe. The arm is then controlled to lift the die up and away from the film and transports it over to a grid carrier where the arm descends to position the die in a slot in the carrier and the vacuum is turned off so the die is placed in the carrier.
Typically the pick-and-place station will comprise a second camera positioned to obtain an image of the grid carrier and computer control for assuring that the dies are placed in the appropriate receptacles in the grid carrier. The die can then be delivered to the next station for further processing.
The die may contain any type of micro electromechanical systems structure, such as optical mirrors. The die may also include other circuitry associated with the micro electromechanical systems structure.
When a MEMS die is conventionally fabricated, the circuitry portion of the MEMS die is typically coated with passivation to protect it. However, the micro electromechanical systems structure cannot be passivated since it must be able to move freely.
A micro electromechanical systems structure is positioned essentially in the center of the MEMS die. Due to the fact that the micro electromechanical systems structure is comprised of extremely small sections of polysilicon so that it is resilient, the micro electromechanical systems structure is extremely fragile and great care must be taken during fabrication, up to and including the final packaging steps, not to damage or contaminate the micro electromechanical systems structure.
If a MEMS wafer comprising a set of micro electromechanical systems structure dies was passed through the standard die separation process as described above, the microstructures would be destroyed. The water jet sprays used in a sawing process would destroy the micro electromechanical systems structure. If any of the micro electromechanical systems structures happen to survive the water spray during the sawing operation, the micro electromechanical systems structure would be destroyed during the subsequent spraying and brushing in the cleaning operation. Further, if any of the micro electromechanical systems structures survived those two steps, the micro electromechanical systems structures would be prone to damage in the pick-and-place station by the vacuum-equipped arm, which picks up the MEMS dies and places the MEMS dies in the grid carrier.
In summary, MEMS structures on the surface of a die are very susceptible to damage and particle contamination at all post wafer fabrication process stages. Protection of the MEMS structures from damage and/or contamination during the sawing process is a critical element of the fabrication process. The MEMS die must then be carefully handled and protected before it is processed at the next step that can be in the same building or geographically remote from the dicing process. The MEMS die is also susceptible to damage at the die attach process where the die is placed on a substrate, carrier, or inside a package.
Accordingly, it is desirable to separate the individual MEMS dies from a MEMS wafer containing a plurality of MEMS dies without realizing the problems of the conventional removal processes. It is further desirable to provide a die separating process that protects micro electromechanical systems structures during a dicing of a MEMS wafer to produce individual MEMS dies. Moreover, it is desirable to fabricate a MEMS wafer that provides protection for micro electromechanical systems structures during a dicing of a MEMS wafer to produce individual MEMS dies.
Furthermore, it is desirable to saw the MEMS dies and to use the same protection device used during sawing or dicing for protection during subsequent process steps with removing of the protection device before interconnection of the die. This facilitates the minimization of the effects of handling of the MEMS die during assembly processing and gives protection to the MEMS die during any processing, handling, or transportation.